Detecting breakdown in ultra-thin dielectrics using a fast voltage ramp

We propose a voltage ramp technique which detects ultra-thin oxide breakdown when other techniques (such as noise) fail. This technique provides a straightforward method of extending conventional ramp breakdown techniques to ultra-thin dielectrics (<3.5 nm). This technique monitors the use-condition current (or current below stress current) after each stress-step in the ramp. We apply this method to a wide range of oxide areas and gate oxide thicknesses. We show for the first time that the post-stress leakage current is independent of oxide area over 7 orders of magnitude and for 5 oxide thickness from 20 nm to 2.3 nm. In addition, we show that the voltage ramp Weibull distribution statistics scale with area and are consistent with constant voltage stress tests on 2 nm gate oxides. We model the post-breakdown I-V characteristics and show a space-charge limited behavior. This observation is used to explain why a modified voltage ramp technique is needed for ultra-thin oxides and defect-detecting large area structures. Finally, we demonstrate that new noise techniques may be necessary to detect breakdown during constant voltage stress in ultra-thin gate oxides.